Newly released data that measures Wall Street’s sentiment is sending out some conflicting numbers.

The Sell Side Indicator, a measure of Wall Street’s bullishness put out by Bank of America Merrill Lynch, has risen to levels last seen in March 2009. That coincided with a market bottom and was a great time to buy stocks.

So why is the data conflicting? Because the indicator is contrarian in nature, meaning when sentiment is rising, stocks tend to fall. When sentiment falls, stocks tend to rally.

Since hitting an all-time low of 43.9 in July, the indicator has now risen to 52.9 in August, up from 52.3 in July. But even though the indicator has risen in 10 of the past 13 months, its current levels are still flashing buy signals, said Savita Subramanian, equity and quantitative strategist for Bank of America Merrill Lynch.

“Given the contrarian nature of this indicator, we remain encouraged by Wall Street’s ongoing lack of optimism and the fact that strategists are still recommending that investors significantly underweight equities at 53% versus a traditional long-term average benchmark weighting of 60-65%,” she said.

Ms. Subramanian points out the S&P 500 has risen some 20% since the indicator bottomed out last July, but said data suggests there’s more upside room for the index.

“History suggests that strong equity returns can last for years after the indicator troughs,” she said.